1. Field of the Invention
This invention relates generally to a solar pond, and more particularly to a saline type (salt gradient) solar pond.
2. Description of the Prior Art
Solar ponds may be generally classified, by the kinds of liquid to be used as the absorbent or heat storage medium of solar energy, into a non-saline type that utilizes homogeneous water (fresh water) and a saline type that utilizes water solutions of various kinds of salt, and also classified by the structures thereof.
Of these types, a saline type solar pond stores salt of various kinds, such as table salt (sodium chloride), magnesium chloride, and boric acid dissolved in a water solution within a relatively shallow pond of approximately 2 to 3 meters in depth. Such a pond establishes a concentration gradient of the solution such that deeper water has a higher concentration and shallower water has a lower concentration, as shown in FIG. 4(b). In this structure, the sun's rays, which are absorbed as heat while penetrating water at water depths corresponding to the respective ray's wavelengths, eventually reach the deepest portion of the pond where all the remaining rays are absorbed. The heated water of the deepest portion never rises because the water of the pond maintains a certain concentration gradient, i.e., a gradient due to specific gravity such that deeper water has a higher specific gravity. All of the water of the pond is thus prevented from undergoing natural convection and, as a result, the water remains heated at high temperatures at lower water depths. This means that the sun's rays are completely converted into heat and absorbed within the water, and the water maintains a certain temperature gradient substantially similar to the concentration gradient of FIG. 4(b).
As described above, a saline type solar pond can obtain higher temperatures as compared to a solar pond of the non-saline type, so that it is most widely used. In general, the salt water solution of the bottom layer (a heat storage layer in FIG. 4(a)) which is heated to the highest temperature is taken out externally so as to be utilized as a heat source.
In the saline type solar pond, it is essentially required to establish a saline concentration gradient corresponding to the respective water depths, and moreover to maintain it all the time during operation. However, this necessitates cumbersome and complicated procedures.
Hitherto, there has been practiced such a method that besides a solar pond, another tank is provided having a salt water solution of a specific concentration and which feeds the same into the solar pond. A solution of lower concentration than the previous one is subsequently prepared in the tank and delivered to the pond on top of the previously fed solution. This procedure is repeatedly performed in several steps so as to sequentially stack layers of solutions of different concentrations until a specified concentration gradient is established. There has also been practiced an extremely simplified method in which salt is directly introduced into a pond so that a salt water solution of high concentration is first prepared, and then fresh water is filled upon this solution so as to cause a natural diffusion.
However, such methods are effective only for a solar pond of a small scale, and in the case of larger practical scales such as a pond having more than several thousand square meters in surface area, the water stored therein generally exceeds ten thousand tons and the salt to be used reaches upward of a thousand tons. As a result, should the above-described methods be employed, a very large tank becomes necessary and, in addition, it takes several months to establish a specified concentration gradient of salt water solution. This inevitably causes restrictions on the scale of solar ponds in terms of economic considerations and time-consuming procedures.
Moreover, once established, the concentration gradient of the salt water solution is gradually lost due to the material diffusion of the salt, thereby making the solution uniform. It is therefore necessary that salt be supplied to the bottom layer of the pond while at the same time, fresh water is sprayed over the surface layer so as to maintain the concentration gradient, and this also necessitates cumbersome and complicated procedures.
Particularly, in the case of conventionally structured solar pond wherein both the intake and the return outlet are fixed in position, when the concentration gradient is destroyed by windstorm or boiling or some other cause, it is almost impossible to repair the concentation gradient so as to recover its original extent in a brief period. Moreover, should this repair be performed, it becomes a large scale and time-consuming procedure in which all the solution in the pond is thrown away, and the gradient is reestablished anew. In addition, should a solution of such a high concentration be discharged from the solar pond into a river, there would be a danger of creating environmental pollution problems.